1. Field of the Invention
This invention is in the field of vapor phase deposition of glass forming compounds on the inside wall of a heated glass tube and is useful in the manufacture of optical glass fibers.
2. Description of the Prior Art
Preforms consisting of chemically pure silica glass or similar glasses are required for the manufacture of optical glass fibers which normally consist of a core and a cladding having different indices of refraction. The glasses can be modified with various dopants such as GeO.sub.2, P.sub.2 O.sub.5 or a silicon fluoride in order to modify the index of refraction. Since the refractive index profile of the fibers that are drawn from the preform is geometrically similar to that of the preform, the preform should already contain the desired profile.
A modified chemical vapor deposition method (MCVD) has been described in Proc. IEEE 62 (1974) pages 1278-1279 as well as in Appl. Optics, Vol. 18 (1979) pages 3684-3693. Both of these references are incorporated herein by reference for purposes of providing background.
Radial refractive index profiles that are matched to the requirements of communications technology can be achieved with this method by means of a layer-by-layer deposition of super-pure glass onto the inside of a cylindrical silica glass tube from the vapor phase. In a typical process, SiCl.sub.4 together with oxygen as well as GeCl.sub.4, POCl.sub.3 or SF.sub.6 are conducted through the tube as dopants and converted into amorphous oxides or fluorides in a hot zone. The oxides deposit downstream on the wall and are fused into glass. Layers of glass of from 10 through 25 .mu.m are typically applied. It would be desirable for reasons of economy to deposit thicker layers per interval of time. This is rendered difficult, however, by the fact that closed pores are frequently formed when sintering or fusing thick powder layers, the closed pores later leading to the formation of bubbles, particularly when collapsing the tube into a rod. It frequently occurs, therefore, that the refractive index profile is destroyed and sometimes even the preform itself. There is an additional difficulty in that the heating of the mixture is no longer entirely successful when higher gas throughputs are used. Another disadvantage is that thick layers lead to an undesired so-called saw-toothed overall profile due to their inherent, unfavorable, inwardly decreasing refractive index profile.
Previous attempts have been made to resolve the problems of bubble formation by modifying the hot zone (see, for example, J. Non-Cryst. Sol. 38, 1980, pp. 831-836). In order to facilitate fusing, the viscosity of the glass can be lowered by means of doping with P.sub.2 O.sub.5 as fluxing agent, but this deteriorates the light transmission properties of the glass. Admixture of helium is also possible to promote the powder consolidation because of its high diffusability. Glass fusing rates higher than about 0.5 g/min can only be realized, however, with accompanying disadvantages.